Catalytically active components can be introduced into zeolite cavities or on their crystal surface by ion exchange, impregnation, codeposition or adsorption from gaseous phase. Because of the nature of zeolites, ion exchange is often used to prepare metal containing zeolite catalysts. This method does not always provide a uniform distribution of small amounts of cations in the bulk of the catalyst. Improved distribution of metals was obtained by exchanging the noble metal cation, i.e. Pd(NH.sub.3).sub.4.sup.2+ in the presence of competing ions such as NH.sub.4.sup.+. U.S. Pat. No. 3,527,835 (Sept. 8, 1970).
A variety of volatile or soluble organic or inorganic compounds are used to load metals on zeolites by adsorption. Carbonyls or hydrocarbonyls (U.S. Pat. No. 3,013,987, U.S. Pat. No. 3,013,988, U.S. Pat. No. 3,013,990), acetylacetonates (U.S. Pat. No. 3,013,987, U.S. Pat. No. 3,013,990), halogenides (U.S. Pat. No. 3,013,987, U.S. Pat. No. 3,013,986), alkyl derivatives (U.S. Pat. No. 3,013,986), and pi allyl species (Kinet. Catal. 13, 1335 (1972)), can be used for this purpose.
Preparation by ion exchange generally involves metal amine complexes (Pd(NH.sub.3).sub.4.sup.2+) because the zeolite is in the ammonium form during ion exchange.
Since it is the metallic state (Pd.sup.o) rather than the ionic state (Pd.sup.2+) which is catalytically active, the metal loaded zeolite is heated to decompose the metal complex. Heating in hydrogen provides a less active catalyst. The unstable, neutral, and hence mobile hydride leads to agglomeration of Pt (Proc. 5th Intern. Congr. Catalysis, Miami Beach, 1972 (1973, 1, 329) as follows: ##STR1##
Prior art teaches that the best distribution of a metal in a molecular sieve can be achieved when water is removed from the zeolite before reduction (U.S. Pat. No. 3,013,982; Proc. 5th Intern. Congr. Catal. (1973) 1, 329). Additionally, teachings indicate that water molecules, which are bound to Pd.sup.2+ cations, prevent their interaction with hydrogen and hence inhibit their reduction to the metallic state (Advan. Chem. Ser. 121, 66 (1973)). Accordingly, state of the art catalyst treatment after ion exchange is generally performed in a dry atmosphere.